1. Field of the Invention
This invention relates to tempering glass sheets, and particularly to glass sheet tempering involving reducing the consumption of energy during the tempering process. Much energy is required in tempering glass sheets because tempering involves first heating glass to an elevated temperature and then rapidly chilling the heated glass. Any reduction in energy consumption required for glass tempering is very important in these days of energy crisis.
The tempering of a sheet of glass involves the purposeful creation therein of a permanent compressive residual stress in the surface portions of glass sheets by rapidly cooling its surfaces from an elevated temperature, thereby producing a rigid envelope, such that upon subsequent cooling of the glass and hardening its central portion, which tends to contract three-dimensionally during said hardening, produces a substantial compressive stress in the surface portions of the glass sheets. This increases the effective strength of the glass sheet, since glass is relatively strong in compression but weak in tension. A glass sheet so tempered exhibits a high residual compressive stress, so that a relatively high and sudden, sharp, tension-generating stress is required to rupture a sheet of glass of this kind.
Tempered glass sheets having a high residual compressive stress in their surface portions are considerably more resistant to fracture on impact with foreign objects, such as road stones and the like, than similar sheets that are untempered. Furthermore, when tempered glass sheets do break, they form relatively small, relatively smoothly-surfaced particles which are substantially less dangerous than the jagged fragments of untempered glass that result when the latter is fractured.
Those skilled in the art of tempering glass sheets have appreciated that the problems involved in tempering thin glass sheets are more severe than those involved in tempering thicker glass sheets. The tempering process depends upon maintaining a temperature gradient through the thickness of the glass such that the central portion of the glass is above its strain point while the surface portions thereof cool to below the strain point to form a rigid envelope. With relatively thin glass, the time in which this gradient can be accomplished is relatively short; so that with relatively thin glass sheets, it is essential to use a very rapid cooling rate, at least during the initial stages of the cooling necessary to produce a suitable temper. Furthermore, when the cooling is done by applying tempering medium in the form of a gas to the surfaces of the heat-softened glass sheet, the gaseous medium must be supplied to the surfaces of the sheet at a very rapid rate.
Blasts of a gaseous cooling medium imparted against the opposite surfaces of a heat-softened glass sheet to effect the temperature gradient required for tempering during the cooling step are very likely to distort the heat-softened surfaces during the initial cooling of said surfaces. Such distortion spoils the optical properties of a tempered glass sheet. If the cooling medium is applied at a slower rate than that which causes surface distortion, the temperature gradient throughout the glass thickness is liable to be insufficient to provide a temper of sufficient magnitude to insure the production of safety glass.
2. Description of the Prior Art
U.S. Pat. No, 2,093,040 to Eckert relates to a tempering process that may be considered as involving two steps. In the first step, the glass sheet is cooled as quickly as possible by cooling medium to a temperature which lies at or near or somewhat below the annealing temperature of the glass to be hardened, i.e., that temperature at and below which the temporary stresses are mainly developed. After attaining this temperature state, the sheet is further cooled by cooling medium at a slower rate. U.S. Pat. No. 3,223,507 to Thomas discloses a two-step operation in which the temperature of the supporting gas and that of the heat source above the glass is lowered immediately before the glass is exposed to cooling medium in the form of the flow of quenching air that is impinged upon the heated sheet to temper it. The Thomas disclosure concerns a two-step heating operation prior to the cooling step.
U.S. Pat. No. 3,847,580 to Misson discloses a two-step cooling operation during tempering of glass sheets to reduce the frequency of glass breakage by conducting the cooling operation in steps of which the first is the most severe and the others are accordingly less severe but still such as not to permit untempering of the surfaces of the glass.
U.S. Pat. No. 3,881,907 to Starr covers a two-stage cooling operation for tempering in which glass is first cooled by cooling medium in the form of gas previously cooled in a vortex tube during a first stage of cooling at a rate of flow sufficient to at least temporarily harden and increase the resistance of the surface of the glass sheet to deformation followed by a second stage of cooling the glass sheet with additional cooling medium in the form of gas not previously cooled.
U.S. Pat. No. 3,883,339 to Michalik and Neely also discloses a multiple-stage cooling operation as part of a glass tempering process in which a sublimable cooling medium is included in the cooling medium applied during a first stage of cooling followed by cooling with a second, less expensive cooling medium, such as air, during the second stage of cooling.
While some of the aforesaid patents resulted in reducing the consumption of energy required to develop a given temper per unit of glass sheet treated, the present energy crisis makes it important to develop further improvements in the glass tempering process over those represented in the aforesaid patents, particularly those that would involve even more efficient use of energy. Furthermore, the application of cooling medium at a relatively rapid rate in the first stage of a multiple-stage cooling operation, believed to be necessary in the tempering of thin glass sheets, causes surface irregularities which provide optical defects in the finished product.